1. Field of the Invention
The present invention relates to a method for manufacturing a solid state image pick-up device, and more particularly, to a method for processing a substrate for improving gettering efficiency on which a solid state image pick-up device is formed.
2. Description of the Related Art
In general, solid state image pick-up devices are used in various fields including public welfare, industry, broadcasting, and munitions, and are applied to various electronic products such as cameras, camcorders, multimedia devices, and surveillance cameras. At present, as electronic products become smaller and the number of pixels of image data electronic products are required to receive and process is increased, demand for an on-chip type solid state image pick-up device having a microlens also increases.
Solid state image pick-up devices include charge coupled devices (CCD) and complementary metal oxide semiconductor (CMOS) devices. The CCD and CMOS solid state image pick-up devices can be integrated onto a silicon substrate.
However, during a process of forming a solid state image pick-up device, or prior to or following such a process, heavy metals such as iron (Fe), copper (Cu), and nickel (Ni) can penetrate into the rear side of the silicon substrate. When heavy metals penetrate into the rear side of the silicon substrate, crystalline defects occur on the silicon substrate, thereby resulting in the possibility of leakage current from the solid state image pick-up device. When such leakage current occurs, a dark level phenomenon causes a display screen to not look completely dark even though no light is detected by the solid state image pick-up device. Alternatively, in a case where there are heavy metal defects on the rear side of the silicon substrate, image defects such as white dots appear on a display screen.
At present, in order to prevent crystalline defects and image defects caused by heavy metal contamination, the substrate in the solid state image pick-up device is gettered. Gettering methods include an intrinsic gettering method and an extrinsic gettering method. The intrinsic gettering method is a method for thermally-processing a substrate at a predetermined temperature to remove defects and contamination. However, the intrinsic gettering method is accompanied by a high-temperature process, thus the intrinsic gettering method is not often used.
Meanwhile, the extrinsic gettering method is a method by which the rear side of a semiconductor substrate is heavily doped with phosphorus (P) ions. Here, due to doping with phosphorus (P)-ions, phosphorus vacancies and dislocation defects occur in the silicon substrate, and heavy metals gather at these phosphorus vacancies and dislocation defects. Here, in general, gettering efficiency is proportional to the concentration of phosphorus ions (P), and solid solubility of the substrate in silicon is defined, thus it is impossible to increase the concentration of P ions beyond the solid solubility of the substrate in silicon. As a result, heavy metal defects in the solid state image pick-up device cannot be removed by the gettering methods, thus image defects are still produced in a screen of the solid state image pick-up device.
To address the above limitations, it is a first objective of the present invention to provide a method for manufacturing a solid state image pick-up device capable of removing crystalline defects and image defects.
It is a second objective of the present invention to provide a method for manufacturing a solid state image pick-up device capable of further improving gettering efficiency by introducing the rear side of a semiconductor substrate with impurities for gettering beyond the solid solubility in silicon.
It is a third objective of the present invention to provide a method for manufacturing a solid state image pick-up device capable of preventing heavy metals from penetrating into a substrate on which the solid state image pick-up device is formed, thereby increasing the lifespan of the solid state image pick-up device.
Accordingly, to achieve the above objectives, according to one aspect of the present invention, there is provided a method for manufacturing a solid state image pick-up device. The method includes the following steps. First, a semiconductor substrate having a front side on which a solid state image pick-up device may be formed, and a rear side opposite to the front side is provided. Next, a polysilicon layer including impurities for gettering having a predetermined concentration is deposited, or otherwise formed, on the rear side of the semiconductor substrate. Last, the thickness of the polysilicon layer including the impurities for gettering is reduced, thereby condensing the impurities for gettering into the reduced polysilicon layer. It is preferable that the step of reducing the thickness of the polysilicon layer is performed by oxidizing a predetermined thickness of the polysilicon layer
In order to achieve the above objectives, according to another aspect of the present invention, there is provided a method for manufacturing a solid state image pick-up device. The method includes the following steps. First, a semiconductor substrate having a front side on which a solid state image pick-up device may be formed, and a rear side opposite to the front side is provided. Next, a polysilicon layer including impurities for gettering having a predetermined concentration is formed on the rear side of the semiconductor substrate. Last, a predetermined thickness of the polysilicon layer including the impurities for gettering is oxidized, and the impurities for gettering are condensed into the reduced polysilicon layer. The step of oxidizing a predetermined thickness of the polysilicon layer is performed so that xc2xd through {fraction (1/10)} of its deposition thickness of the polysilicon layer may remain.
In order to achieve the above objectives, according to still another aspect of the present invention, there is provided a method for manufacturing a solid state image pick-up device. The method includes the following steps. First, a semiconductor substrate having a front side on which a solid state image pick-up device may be formed, and a rear side opposite to the front side is provided. Next, a polysilicon layer including impurities for gettering having a predetermined concentration is deposited on the rear side of the semiconductor substrate. Next, a predetermined thickness of the polysilicon layer including the impurities for gettering is oxidized, and the impurities for gettering are condensed into the reduced polysilicon layer. Last, a protective layer is formed to cover the rear side of the semiconductor substrate.
It is preferable that the step of oxidizing the polysilicon layer to a predetermined thickness is performed by oxidizing the thickness of the polysilicon layer so that xc2xd through {fraction (1/10)} of the entire thickness of the polysilicon layer may remain, and the impurities for gettering are phosphorus (P) ions, or arsenic (As) ions.
It is also preferable that the step of forming a polysilicon layer including impurities for gettering on the rear side of the semiconductor substrate includes the steps of depositing an intrinsic polysilicon layer on the rear side of the semiconductor substrate, and introducing the polysilicon layer with the impurities for gettering. It is also preferable that the step of introducing the polysilicon layer with the impurities for gettering is performed by ion implantation, doping, or diffusion.
It is also preferable that the step of forming a polysilicon layer including impurities for gettering, on the rear side of the semiconductor substrate is performed by depositing the polysilicon layer in which the impurities for gettering are doped, on the rear side of the semiconductor substrate by a chemical vapor deposition (CVD) method.
It is also preferable that the step of forming a polysilicon layer including impurities for gettering, on the rear side of the semiconductor substrate is performed by introducing the impurities for gettering to the solid solubility in silicon into the polysilicon layer, and the polysilicon layer has impurities concentration beyond the solid solubility in silicon by step for reducing the thickness of the polysilicon layer.
It is also preferable that the protective layer is formed of silicon nitride (Si3N4) or silicon oxynitride (SiON).